The invention relates to apparatus and method for producing hard copy of an optical image displayed on a faceplate of a raster scanned cathode ray tube (CRT) display device.
It is recognized that in office system terminals of the future, the need for local printing will be a requirement even though most operations will be executed by interacting directly with information displayed on a display device. Current stand-alone printers which operate to produce hard copy output directly from digital information supplied, for example from a control unit, are fast and produce high quality print on ordinary paper but are expensive. Low utilization of such devices as output printers for display terminals tends to make them uneconomic.
Hard copy of images displayed on interactive display terminals of the type which rely on the storage tube principle for their operation can be obtained using specially designed stand-alone printing apparatus. Such printing apparatus requires a tailor-made CRT with a flat cross-section tube which scans only in one dimension. In order to print an image displayed on the storage tube faceplate, the image is interrogated with a read beam which is raster-scanned across the faceplate. A stream of analogue pulses representing the lit portions of successive scanned rows of the stored image are produced from the storage tube faceplate. These pulses are supplied to the intensity control of the printer CRT which is controlled to scan in synchronism with the line-by-line scan of the read beam raster. Thus, the entire image is reproduced one raster scan line at a time on the elongated faceplate of the printer CRT. A paper feed mechanism in the printing apparatus draws light sensitive paper over the CRT faceplate at a rate corresponding to the rate of progression of the raster scan across the stored image. By this means, the paper is exposed line-by-line by the lit portions of the printer CRT and a latent image corresponding to the displayed image is generated in the paper. The exposed paper is then fed through a developing station in order to develop the latent image.
The high cost of storage tube CRT's makes them unacceptable for such high quantity use as is required in an office system environment and accordingly the terminal displays are usually provided by conventional raster-scanned CRT's. A need exists, therefore, for printing apparatus by which a terminal operator can obtain a hard copy of an image displayed on a raster-scanned CRT terminal promptly and cheaply.